1. Field of the Invention
This invention relates to a deposited-film forming process and a deposited-film forming apparatus. More particularly, it relates to a deposited-film forming process and a deposited-film forming apparatus in which deposited films are formed on a belt-like substrate (hereinafter, the belt-like substrate is referred to as a xe2x80x9cwebxe2x80x9d) while lengthwise continuously transporting the web in a vacuum chamber. Still more particularly, it relates to a deposited-film forming process and a deposited-film forming apparatus that are preferably applicable in a roll-to-roll system suited for the continuous formation of photovoltaic devices, i.e., solar cells.
2. Related Background Art
In recent years, generation of electricity using sunlight by making use of solar cells has attracted notice as an environmentally safe, clean system, and a variety of research and development has been made so that solar cells can be put into practical use.
In order to make solar cells practical, it is required for them (1) to have a sufficiently high photoelectric conversion efficiency, (2) to have characteristics which are stable with time and (3) to be feasible for mass production. Assuming that it is necessary for usual homes to be supplied with an electric power of about 3 kW peer household, the solar cells must have an area of about 30 m2 if all the power is supplied by solar cells having a photoelectric conversion efficiency of about 10%. Thus, in order to supply the electric power necessary for 100,000 homes, the solar cells must have an area as large as 3,000,000 m2.
Under such circumstances, amorphous silicon solar cells that can be produced at a lower cost than solar cells produced using single-crystal silicon attract notice. This is because material gases such as silane which may be decomposed by glow discharge so that semiconductor thin films are deposited on a relatively inexpensive substrate such as a glass or metal sheet.
Accordingly, various proposals have been made for the process and apparatus for producing amorphous silicon solar cells.
For example, U.S. Pat. No. 4,485,125 discloses a continuous plasma-assisted CVD apparatus employing a roll-to-roll system as shown in FIG. 1.
As shown in FIG. 1, in this continuous plasma-assisted CVD apparatus 1100, film forming chambers 1101 to 1106 are arranged in a straight line as viewed from the top and arranged in a catenary as viewed from the side. (FIG. 1 is a diagrammatic cross-sectional side view). As a substrate for film formation, a belt-like substrate (web) 1107 is used which has a desired width and is sufficiently long.
Inside the film forming chambers 1101 to 1106, glow discharge regions in which desired semiconductor layers are formed are provided, the regions being respectively surrounded by the web 1107 and high-frequency electrodes 1108 to 1113. The web 1107 is transported so as to be successively exposed to all the glow discharge regions.
At connecting portions between the respective film forming chambers, gates 1114 called gas gates are provided to separate the chambers by spaces. The gas gates 1114 isolate materials gases fed into adjoining film forming chambers to prevent their mutual diffusion and also allow the web to pass through them. Because of the presence of these gas gates 1114, the material gases in the desired film forming chambers are not reciprocally moved when the web 1107 is successively transported to the adjoining film forming chamber, and hence semiconductor layers with the desired conductivity types can be deposited.
U.S. Pat. No. 4,440,107 also discloses a magnet roller that supports and transports the web in each film forming chamber, and a radiation heater for the magnet roller.
The magnet roller is internally provided with a permanent magnet and magnetically hangs a web made of a ferromagnetic material. This magnetic roller is appropriately disposed, whereby the web can be transported while being supported at substantially the same plane.
The radiation heater can heat the web to the desired temperature. Upon drive of this continuous plasma-assisted CVD apparatus 1100, the web 1107 is lengthwise continuously transported, while the desired semiconductor layers are formed on the web by glow discharging, so that the semiconductor layers are superposingly formed on the web 1107 and desired semiconductor junction devices can be continuously formed. As a result, it becomes possible to mass-produce semiconductor junction devices (e.g., solar cells) with a large area. Thus, the roll-to-roll system is a process suited for the mass production of solar cells.
Meanwhile, plasma processing making use of microwaves has attracted notice. Microwaves have a high frequency and hence energy density can be made higher than when conventional radio frequency (RF) waves are used. Thus, the microwaves are suited for generating and maintaining plasma in a good efficiency.
For example, U.S. Pat. No. 5,510,151 discloses deposited-film forming process and apparatus of a roll-to-roll system employing microwave plasma-assisted CVD. Deposited films can be formed even at a low pressure when plasma is generated using microwaves. Also, active species that may cause a lowering of characteristics of deposited films can be prevented from being polymerized, so that not only deposited films having a high quality can be obtained but also powder of polysilane or the like can be kept away and also the rate of film formation can be dramatically improved, as so reported therein.
In order to mass-produce solar cells using the belt-like substrate (web) transport means comprising the roll-to-roll system, the web may be made longer and wider. When, however, it is made longer and wider, a problem may occur such that the web deforms because it is heated to a desired temperature by a heater for the substrate or heated by the supply of heat from plasma. In the case of the continuous plasma-assisted CVD apparatus employing the web transport means comprising the roll-to-roll system, the web may undergo thermal deformation. The deformed part comes into contact with opening adjuster plates for forming openings that communicate between chambers, as will be described later, so that the web may have scratches on its film forming surface to make film formation faulty at that part, to make the devices faulty at that part or to cause a break of the semiconductor junction devices at that part.
In a case where the web is transported in such a way that it forms a cover of a discharge box, the thermal deformation of the web may cause a gap between the web and other members constituting the discharge box, so that the plasma shut up in the discharge box may leak to make discharge unstable.
An object of the present invention is to solve the above problems and provide a deposited-film forming process and a deposited-film forming apparatus that enable mass production in a high yield without causing scratches on the film forming surface (i.e., surface for forming a deposited film).
Another object of the present invention is to provide a deposited-film forming process and a deposited-film forming apparatus that enable stable discharge and can continuously form deposited films having uniform quality and uniform thickness.
Still another object of the present invention is to provide a deposited-film forming process comprising continuously transporting a belt-like substrate (web) in the length direction of the substrate so as to form a part of a discharge space, and continuously forming a deposited film on the web, wherein the web is transported while bringing the transverse sectional shape of the web which forms a part of the discharge space into a curved shape as viewed crosswise with respect to the direction of transport of the web.
A further object of the present invention is to provide a deposited-film forming apparatus comprising a web which is continuously transported lengthwise, means for transporting the web so as to form a part of a discharge space, and means for continuously forming a deposited film on the web, wherein the means for transporting the web comprises a roller for bringing the transverse sectional shape of the web which forms a part of the discharge space into a curved shape as viewed crosswise with respect to the direction of transport of the web.